Dual pulse frequency compounded super harmonic imaging for phased array transducers

Second harmonic imaging is currently the standard in commercial echographic systems. A new modality, super harmonic imaging (SHI), is based on combining the 3rd to 5th harmonic generated during sound propagation in tissue. This emerging modality could further enhance resolution and quality of echographic images. To meet the bandwidth requirement for SHI an interleaved phased array was developed. Array elements used in transmission generally have bandwidths of ~ 80% leading to gaps between harmonics in the spectral domain. This causes ripple artifacts in the echo image. Last year we introduced a new dual pulse frequency compounding method to reduce these artifacts and showed initial single element results [1]. In this work we implement and optimize the dual pulse method for an interleaved array on an ultrasound system and research its imaging characteristics, i.e. point spread functions (PSF). In the dual pulse SHI method each trace is constructed by the summing of two firings, the second slightly frequency shifted compared to the first. To study the dual pulse method´s performance an interleaved array (44 1 MHz and 44 3.7 MHz elements, optimized for echocardiography) was used in combination with a fully programmable ultrasound system. Initial estimates for the frequencies of the first and second pulses as well as the pulse duration were optimized experimentally. Our findings confirm that the transfer functions of both transducer and system have to be taken into account to determine the optimal transmission frequencies for the dual pulse SHI method. Moreover, a trade off exists between dual pulse signal length and peak intensity. The optimal results with the dual pulse technique were achieved using a transmission length of 2.5 cycles and transmission frequencies of 0.87 MHz and 1.12 MHz. The lateral beam widths of the optimal dual pulse signal are 1.2 times smaller at the -6 dB level and equal at the -20 dB level compared to the third harmonic. The axial beam widths- of the optimal dual pulse signal are 3.1 times smaller at the -6 dB level and 1.6 times smaller at the -20 dB level compared to the third harmonic. Not only does dual pulse method solve the ripple artifacts associated with imaging using multiple harmonic bands, dual pulse SHI has markedly improved axial and lateral resolutions compared to the third harmonic at higher than second harmonic intensities.